GREEK SYMBOLS COMMONLY USED
IN MR IMAGING
and their transcriptions in the Glossary


(alpha)
flip angle


(gamma)
gyromagnetic ratio


(delta)
chemical shift


(mu)
vector of the magnetic moment,
and


(mu)
permeability


(nu)
frequency


(pi)
ratio of the circumference of the circle to its diameter


(rho)
proton density


(tau)
time delay (TE/2)


(tau c)
correlation time


(chi)
susceptibility


(omega)
Larmor frequency
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L

LAO - left anterior oblique

Larmor equation - states that the frequency of precession of the nuclear magnetic moment is proportional to the magnetic field.
omega0 = -gamma · B0 (radians per second); or
nu0 = -gamma · B0/2pi (Hertz)
where omega0 and nu0, respectively, are the frequency, gamma is the gyromagnetic ratio, and B0 is the magnetic induction field. The negative sign indicates the direction of rotation.

Larmor frequency (omega0 or nu0) - the frequency at which magnetic resonance can be excited; given by the Larmor equation. By varying the magnetic field across the body with a gradient magnetic field, the corresponding variation of the Larmor frequency can be used to encode position. For protons (hydrogen nuclei), the Larmor frequency is 42.58 MHz/Tesla.

Lattice - the magnetic and thermal environment with which nuclei exchange energy in longitudinal relaxation.

Line imaging —> Sequential line imaging.

Line scanning —> Sequential line imaging.

Line width - width of line in spectrum; related to the reciprocal of the transverse relaxation time (T2* in practical systems). Measured in units of frequency, generally at the half-maximum points.

Localized spectroscopy - the acquisition of spectra from a region within a sample. The region is usually defined using selective RF pulses and field gradients.

Longitudinal magnetization (Mz) - component of the macroscopic magnetization vector along the static magnetic field. Following excitation by RF pulse, Mz will approach its equilibrium value M0, with a characteristic time constant T1.

Longitudinal relaxation - return of longitudinal magnetization to its equilibrium value after excitation; requires exchange of energy between the nuclear spins and the lattice.

Longitudinal relaxation time —> T1.

Lorentzian line - usual shape of the lines in an NMR spectrum, characterized by a central peak with long tails; proportional to 1/[(1/T2)2 + (n-n0)2], where n is frequency and n0 is the frequency of the peak (i.e. central resonance frequency).

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